In the cochlea, acoustical, mechanical and electrical phenomena interact in a carefully coordinated fashion to filter incoming acoustical energy and convert it to neural input. We seek to develop a hierarchy of mechano- electrical-acoustical (MEA) mathematical models that predict the activity and nonlinearity seen in the healthy cochlea. The MEA models include the coupling of the cochlear electric pathways (through the scalae and organ of Corti circuitry) with the outer hair cells (OHCs) and a hydromechanical model for the cochlea. In this grant application, the development of these models is tied to the testing of the following three hypotheses of cycle-by-cycle active processesfor cochlear activity: Hypothesis 1 is based solely on somatic OHC motility; Hypothesis 2 solely on OHC hair bundle (stereocilia) motility and Hypothesis 3 on a combination of OHC somatic and hair bundle motility. Novel tests of these hypotheses using combined electrical and acoustical excitation will be used to determine their validity. We will relate predictions to physiological experimental results and test hypotheses of cochlear activity in collaboration with two experimental groups, at the Oregon Hearing Research Center at Oregon Health and Science University and at the Kresge Hearing Research Institute at the University of Michigan. The ability to predict the response of the cochlea to electrical and mechanical (acoustical) inputs will assist in the development of noninvasive tools for assessing the health of hearing. Such models will also provide a insight into hearing protection by determining mechanisms of damage, and provide guidance in the development of future prosthetic devices.